Vacuum tube and method of manufacture



Aug; 4, 1931. T E. Y. ROBINSON 1,817,445

I VACUUM TUBE AND METHOD OF MANUFACTURE Filed-Dec. 29, 1925 ix 9 Y 1' Iwnmzssss: INVENTOR $14 V5 9 Emeaf )omanEabmsmz W ATT oRNEY Patented Aug.4, 1931 UNITED. 'sTATEs PATENT OFFICE EBNES'IYEOMAN ROBINSON, OF LYMNENGLAND, ASSIGNOB 1'0 ASSOCIATED ELEG- TRICAL INDUSTRIES LIMITED,BRITISH COMPANY vacuum TUBE an!) METHOD or MANUFACTURE Application filedDecember 29,

This invention relates to vacuum electric tube devices and particularlyto the introduction into such devices of vaporizable substances orreagents such as may be employed 6 as getters for the removal ofresidual gases or more especially for increasing the electron emissionfrom cathodes. In this latter connection alkali metals have beendistilled into the tube, the theory being that a molecular 1 layer ofthe alkali metal is formed upon the surface of the cathode so that theelectron emission and efliciency is increased by reason ofthe fact thatthese metals have low work functions and consequently permit the cathodeto be operated at low temperatures.

. Caesium is a notable example of a metal of this class. Employingcaesium the emission from a tungsten'filament at 900 K, is aboutone-third ampere per sq. cm. of cathode surface.

Referring more particularly to caesium this metal 1s very readilyoxidizable so that it cannot be handled and directly introduced into thevacuum electric tube, and the process usually adopted is to heat amixture 0 caesium chloride and calcium in an auxiliary glass'bulbforming an extension of the main envelope of the tube, thereby producingmetallic caesium which is distilled into the main envelope after thelatter has been evacuated and is still in connectdon with the evacuatingpump, the auxiliary bulb being subsequently removed. Caesium, however,at-

tacks ordinary lead glasswhen the temperal J efiect the reaction in abulb of special glass such as that known under the registered Trade Markas Pyrex glass. Furthermore it is necessary that the main tube alsoconsists of a special glass of this nature since it must be sealed tothe auxiliary bulb. In any case the heating of the caesium chloride andcalcium needs care and skill, andfor these reasons vacuum electric tubesof this class have only been rarely employed heretofore.

ture is'above normal so that it is necessary to.

1925, Serial No. 78,124, and in Great Britain December 31, 1924.

exhaustion of the envelope, the compound or the substance is heatedindependent y of the envelope, such for example as by means of a heatingcoil through which an electric current may be passed, or by heat arisingfrom the heat treatment of an electrode, but most conveniently by highfrequency induction in which case the material to be heated may receiveheat from an electrically conducting auxiliary container therefor, or itmay be directly heated by high frequency induction. By this method thesubstance will not bef come oxidized and since the main glass envelopeis not itself directly heated any glass which is suitable for containingcaesium at normal temperatures may be employed for the main envelope.The auxiliary container for the substances may be permanently located inthe main bulb itself or within an extension of the main bulb which canbe subse: v quently sealed off from the latter and removed.

In carrying out the invention according to the preferred method the mainenvelope of the tube consists of ordinary glass and contains for exampletwo or more electrodes supported from the pinch of the tube andconsisting for instance of a tungsten cathode, an anode and *a grid withappropriate lead-in conductors. The envelope is evacuated in the usualway through a tubulation and the auxiliary container such as a nickelbucket for the reagents which comprise a mixture of caesium chloride andcalcium is located in an auxiliary bulb or chamber connected with themain envelope by means of a-smaller tubulation and'also consisting ofordinary glass. After the usual evacuation and the removal of occludedgases from the envelope and electrodes, and while the envelope is still,connected with the exhaustin pump the metal bucket is heated by highErequency in duction so that reaction ensues and the resulting caesiumis distilled into the main envelope, the walls of which arecomparatively cool so that the caesium will condense on to the innersurface thereof without condensing in any appreciable quantity upon thesurface of the auxiliary bulb the which are already warm by reason ofthe heat induced in the metal bucket by high frequency currents. Hencethe caesium does not 7 attack the. glass to an undesirable extent. Whensuflicient caesium has been distilled into the main envelope theauxiliary bulb is sealed off at the aforementioned tubulation.

In a modification of the above method the metal bucket disposed withinthe main en velope itself, for example bymounting it' upon one of thesupporting members of the electrodes or upon one of the electrodes ofthe vacuum electric tube device, in which case the bucket may be heatedby high frequency currents or otherwise, and is not removed from themain envelope after the caesium has been formed. 1 v

In a further modification the caesium may be sealed into a glass capsulewhich is placed 'within.the main envelope or an extension thereof andheated. The capsule is preferably'disposed in a metal container andheated by high frequency induction or otherwise. Such heating causes thecaesium to volatilize and to penetrate the glass capsule and thus toescape into the main envelope, the glass capsule being retained withinthe latter or removed with the extension. The glass capsule should beformed of a glass such as boro-silicate glass which does not easily re-.

- act with caesium.

To enable the invention to be clearly understood and carriedinto elfect,it will now be described with reference to the accompanying drawings inwhich Figs. 1, .2 and 3 inclusive are elevations of vacuum electric tubedevices, illustrating different methods of introducing the caesium orother vaporizable substance in accordance withgjifhe invention.

Fig. 4 is a perspective view to a larger scale of an electrode similarto the electrodes shown in the preceding figures and illustratinganother method of introducing the caesium or other substance. Fig. 5 isasectional view 'on a larger scale of a detail, and Fig. 6 is a viewillustrating a method of producing 5 sealed capsules containing. caesiumor other walls of.

auxiliary bulb may be dispensed with and the substances which is to beintroduced into a vacuum electric tube device.

Referring first to Fig. 1 of the drawings, a three-electrode valve isillustrated comprising a tubular anode 1,- within which is arranged agrid, only the support 2 of which is visible, and a filament thesupports 3 and 4 ofwhich are shown. The several electrode supports aresecured in a pinch 5 formed on a flange tube 6 which is fused in thewellknown manner to the glass envelope 7. A tube 8 is fused into thepinch 5 in connection with-a blow hole 9 therein. The envelope 1 isadapted to be evacuated through the tube 8 which is connected to thepump manifold. The opposite end of the envelope 7 is provided with atubular extension 10 sealed at its free end 11 and provided with arestriction at 12 so that it may be readily sealed off and removed fromthe main envelope 7. The. sealed end 11 is in the form of a pinch inwhich is secured a wire 13 to which is'attached a nickel tube 14 havingits ends closed by pinching or otherwiseand containing a quantity of amixture of caesium chloride and, calcium. The nickel tube. 10 ispreferably 'seamlessor if seamed its'adjacent edges should be Weldedtogether so that highfrequen'cy currents may circulate around it wheninduced therein.

The vacuum tubeas constituted is exhaust'ed in the usual way, theenvelope being ovened for about 10V to 12 minutes at 400, for example,after which the electrodes may be heated by high frequency induction tored heat. The filament may be heated by the passage of a current throughit. After the absorbed gases have been removed upon heat treatment thenickel tube 14.- may be heat sium chloride and the calcium takes placeto produce caesium metal and calcium chloride. The metallic caesiumdistills into the main envelope 7 and condenses on the inner surfacethereof and on'the surfaces of the electrodes. Little or no caesium willcondense uponthe inner surface of the extension 10 since the latter willbe warm by radiation of heat from the nickel tube 14., or in any casethe extension 10 may be warmed by means of the sealing off torch orotherwise-and the caesium thus driven'into the main envelope.

When the caesium has been driven into the main bulb 7 the extension 10may be sealed off at the constriction 12, after which the main bulb maybe sealed off from the pump.

It will be understood that the process above described may be modifiedin various ways, for example,'the caesium maybe distilled into the mainbulb 7 after the latter has been evacuatedgand disconnected from thepump. -0rthe heat treatment of the electrodes and the heating of thetube 14' may be effected after the valve has been evacuated 'the upperen being evacuated through the tabulation 15 at the up by the pumps anddisconnected therefrom.

Referring to-Fig. 2 the nickel tube 14 instead of bem arranged in anextension of the main enve ope 7 is attached to one of the electrodesupports, namely the support 3 for of the filament, the envelope r endof the main bulb, instead of throng a'tube connected with the pinch 5.The nickel tube 14 may be welded to the filament support 3 or-clippedthereto. After the exhaustion and heat treatment of .the bulb,the anode and nickel tube 14 maybe heat treated, either before or aftersealing off chloride into a paste with water and then painting it on tothe magnesium. When calcium is employed a non-aqueous solvent, such asether, may be employed. When the anode.

is heat-treated the magnesium or calcium will reduce the caesiumchloride so thatcaesium 'will be vaporized into the envelope 7. Insteadof securing a. piece of magnesium or calcium to the anode as describedin connection with Fig.3 a nickel pocket 17 may be welded to the anodeas shown in Fig. 4, for the reception of a mixture of caesium chlorideand calcium. 7

In all cases wherein the reagents are secured to or supported upon theanode or are heated by heat generated in the anode, the necessary heatmay be alternatively eifected by bombardment of electrons from the,filament. In cases where the inter-electrode gap is short or wheresurface area of th'eanode is relatively small compared to that of thefilament, the anode may be heated to a suflicient temperature merely byradiation of heat from the filament which, in the case of tungsten, maybe heated to a temperature of 'say 2500? K. 1

When the reagents are heated within the main bulb and a moderatelyvolatile reducing agent such as magnesium or calcium is employed it willbe appreciated that these reducing agents may also be present in" thebulb and serve as getters for the removal of residual gases. j

Instead of introducing the caesium into the bulb in the form of areducible compound as described in connection with Figs. 1 to processthe envelope 7 is connected to the pump manifold by the tube 8 andexhausted while the bulb is ov'ened, the electrodes being next heattreated by a high frequency induction, after which the metal tube orbucket 14 is heated by high frequency induction, which causes thepressure of the metallic caesium in the capsule 18 to rise and at thesame time weakens the walls of the capsule until eventually the caesiumperforates the capsule and passes into the main bulb, after which theextension 10 may be sealed off at the restriction 12. Alternatively theelectrodes ma be heat treated after the main bulb is sealed ofi from thepump manifold, as also may the metal tube or bucket 14. In this case theextension 10 may be removed from the bulb after the latter has beensealed 0E from the pumps and after the caesium has been driven into themain envelope.

In some cases the metallic caesium in the capsule 18 may be heateddirectly by high frequency induction, in which case the nickel tube 14can be dispensed with. The capsule can be located at other places in theenvelope and heated in other; ways.

Capsules. such asi 18 containing metallic caesium may be prepared in thefollowing manner, reference now being made to Fig. 6. A long thin tube20 of boro-silicate glass is provided at one end with a bulb 21containing a mixture of caesium chloride and calcium, the other end 22of the tube 20 being connected with a vacuum pump (not shown),

the tube being preferably heat treated for the removal of occludedgases. The reagents in the bulb 21 are then heated so that they reactwith one another, caesium being produced and distilled into the tube 20,wherein The bulb 21 is then sealed off itcondenses. from the tube 20 at23. A'portion of the tube 20 is then sealed-ofl' at 24, another portionat 25 and so on, each portion thus sealed off containing a littlecaesium, these sealedofi' portions or capsules being adapted to beinserted in the metal tube or bucket 14 for the 'purposeof introducingvaporized caesium into vacuum electric tube devices in the manneralready described.

It will be understood that various detail modifications may be madewithout departing from the scope of the invention, and that while theinyention has been described primarily for the introduction of caesiuminto vacuum electric tube devices for increasing the electron emissionof cathodes, it can be ERNEST YEOMAN ROBINSON.

